SS Vessel with CS attachments (on SS pads), issues??

[julesomar]

What is the potential for reduced integrity of a SS 304/316 pressure vessel with SA516 support lugs, legs, etc. (welded to a a SS pad on the shell)? I thought vendors would typically use SS supports when the amt. of material is small compared to the plates / flanges however I am seeing a fair bit of carbon steel attachments being used. Is there any benefit to using CS for the supports? Does the operating temperature / pressure of the vessel affect using the dissimilar steels?

Thanks,
Jules EIT

 

[metengr]

Jules;

What is the potential for reduced integrity of a SS 304/316 pressure vessel with SA516 support lugs, legs, etc. (welded to a a SS pad on the shell)?

You stated that you have stainless steel pads that will provide for the CS attachments. You did not mention the filler metal that was used nor the service temperature. I would verify the filler metal that was used for the dissimilar metal weld joints only. If it is 309 ss filler metal and the service temperature of the vessel is at or above 800 deg F, you can have a local integrity problem with the weld fusion zone along the carbon steel side of the weld joint over time. If this dissimilar metal weld joint is exposed to temperatures at or above 800 deg F, I would use Inconel filler metal (82 or 182).

 

[gr2vessels]

julesomar,
As usually, metengr has replied comprehensibly, nothing much to ad.
You could save a good buck replacing the less strong SS material with stronger carbon steel (eg.: cheaper carbon steel, less thickness, less cost), for legs, lifting lugs, even skirts for vertical equipment. As above, the welding of dissimilar materials is not a fabrication problem anymore, is rather a corrosion problem, if unprotected. As far as the operating / temperature concerns, you select a suitable welding procedure and all will be fine.
cheers,
gr2vessels

 

[MJCronin]

Jules,

There has been some discussion about these issues on this and other forums.

In the fabrication industry, the commonly used term for these pads are "poison pads"

-MJC

 

[eliebl]

Please correct me If I am wrong but another advantage in using standard carbon steel is that the allowables are based upon tensile and not yield. I have seen stainless cargo transport tanks where over time (several years) there would appear to be growth (creep) of the vessel. This may be due to other factors but there is general note G5 in ASME II-D which sort of implies something similar.

If thermal gradients and thermal expansion are an issue then that would lead me to choose stainless attachments. Also comes down to a cost issue.

EJL

 

[waskillywabbit]

Usually it all boils down to money.

Brian

 

[jte]

eliebel-

Carbon steels may be limited by either yield or ultimate. Depends on which flavor of steel, which code and at what temp. For SA516-70, VIII Div. 1 applications are limited by ultimate through 500°F, then by yield, then by time dependent properties (creep). Using the old Div. 2, SA516-70 was governed by ultimate at low temp, then jumps to governed by yield by 200°F. Allowable stresses for the new Div. 2 are governed entirely by yield until time dependent limits kick in at 800°F.

jt

 

[vesselfab]

As it often happens, we as a fabricator take a completely different approach. If a Stainless Steel vessel is requiring a significant number of attachments, we will make them stainless steel if possible. If you are welding on ""poison pads"" just to isolate the carbon from the heads/shell, why not just make the clip stainless and forget about the disimilar weld problems and the extra work putting on isolation pads. If the pads are required due to loads, then they have to be there, and we still make the clips out of stainless steel just to keep from having to paint the carbon steel. This is what we would rather do. But there is a chemical company that specifies and requires that you place carbon steel pads and clips on their stainless steel vessels.